Automatic tone control



Feb. 20, 1934.

W. S. BARDEN AUTOMATIC TONE CONTROL Filed April 15, 1931 MEANS .SOIIRLT0F 5/5/4541 ENE/P6 Y N m n R Q/W N O E mm A IIIL I W Patented Feb. 20,1934 UNITED STATES PATENT OFFICE 1,947,822 AUTOMATIC TONE CONTROL.

Application April 15, 1931. Serial No. 530,268

8 Claims. .(Cl. 250-20) My present invention relates to radio receiversemploying gain control, and more particularly to an improved method of,and means for, maintaining a predetermined background noise level inreceivers utilizing automatic gain control arrangements. Subject-matterconcerning the relation between attenuation and background noise leveldisclosed in this application, and common to application Serial No.545,704 of G. C. Beers, filed June 20, 1931, is not claimed herein, butis claimed in the said Beers application. 7

The use of automatic volume control devices in radio receivers hasgreatly alleviated the problem of maintaining'a desired signalreproduction level during reception. However, it has been recognizedthat the employment of such devices simultaneously results in anincrease in the background noise level during certain periods ofreception. For example; when thereceived field strength is not highrelative to the noise level, the inherent action of the gain controlwill tend to raise the latter. Again, when no signals are beingreceived,background noises are greatly amplified, and appear to detract from theadvantages otherwise secured by means of the automatic gain control.

It has been found that theparticuiarly annoying background noisescomprise high audio frequencies. This fact can be utilized to advantagein contending with background noises in automatic gain controlreceivers. I have found that if the high audio frequencies beautomatically attenuated when the received field strength is' not highrelative to the noise level, that background noises are not apparent tothe listener.

Accordingly, it is one ofthe main objects of my present invention toprovide a method of, and means for reducing the reproduction ofbackground noises in radio receivers equipped with automatic gaincontrol arrangements whenever theratio of received field strength tobackground noise level reaches a predetermined value, the methodconsisting in attenuating'the relatively high audio frequencies inaccordance with variation in signal field strength.

Another important object of the present invention is to provide incombination with an amplifier, an arrangement for automaticallyattenuating the relatively high audio frequencies, little or noattenuation being introduced at signal field strength exceeding. apredetermined value, the arrangement comprising apath adapted to by-passthe high audio frequencies in accordance with signal field strengthvariation.

Another object of the present invention is to provide in'combinationwith a radio receiver comprising a radio frequency amplifier and adetector, an automatic volume control circuit adapted to maintain thegain of said radio frequency amplifier at a predetermined level, andadditional means comprising a path designed to by-pass high audiofrequencies, the path including an electrical element whose impedance isvaried automatically in accordance with the functioning of saidautomatic volume control circuit. 7

Still another object of the present invention is to provide incombination with a radio receiver comprising a radio frequency amplifierand a detector, an arrangement for automatically disoriminating againstpredetermined audio frequencies in accordance with received signal fieldstrength.

Still other objects of the invention are to improve generally theefiiciency of radio receivers equipped with automatic gain controldevices, and to particularly provide a receiver of this type which isnot only reliable in operation, but economically assembled.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims, the inventionitself, however, as to both its organization and method of operationwill best be understood by. reference to the following descriptiontaken. in connection with the drawing in which I have indicateddiagrammatically one circuit organization whereby my invention may becarried into effect. a

'Referring to the accompanying drawing there is shown a radio receiverincluding a device 1 functioning as a source of signal energy. Thedevice 1 may be, for example, an antenna system, and may be coupled, asat M, to the tuned input circuit of a radio frequency amplifier tube 2.The latter is a space discharged device, and while shown as comprisingthree electrodes may obviously be of the screen grid type if desired.The input circuit of the tube is tuned by a variable condenser 3, whilethe anode, or output, circuit of the tube includes a source of anodepotential B. A resistor 4 in the grounded side of the'cathode produces,in a manner well known to those skilledin the art, bias for the controelectrode of the tube 2. While a single stage of tuned radio frequencamplification has only been shown, it is to be clearly comprise one, ormore, preceding stages of radio understood that the device 1 can readilyam 1 rents.

frequency amplification. The output circuit of the tube 2 is coupled, asat M, to the input circuit of a detector stage. The latter comprises aspace discharge tube 5 whose input circuit is tuned by means of avariable condenser 6. The conventional capacity 7 and grid leak resistor8, both disposed in series with the control electrode of the tube 5,provide detection. The anode circuit of the detector tube includes asource of anode potential B.

The anode circuit itself, is coupled, as at M, by an audio frequencytransformer to any well known type of utilization means. The latter maycomprise one, or more, stages of audio frequency amplification, and asucceeding loud speaker, head phones, or any other well known type ofreproducer. As is well known to those skilled in the art, the rotors ofthe variable condensers 3 and 6, as well as the rotors of any tuningcondensers in stages of amplification prior to the tube 2, may bearranged for mechanical unicontrol, as conventionally represented by thedotted lines designated by the numeral 9.

In order to maintain a predetermined radio frequency amplifier gainlevel there is usually provided an automatic gain, or volume, controlarrangement. The latter, in one of its well known forms, comprises aspace discharge dBVl-Cu 10 having its input electrodes connected to theinput circuit of the detector tube. A source of bias C is connected tothe control electrode of the device 10, while a source of anodepotential B" is connected to the anode.

A resistor 11 is connected between the grounded side of the cathode ofdevice 10 and the negative terminal of the source B". As is well knownto those skilled in the art, the grids of one, or more, of the radiofrequency amplifier tubes are connected to the anode circuit of thecontrol tube 10. Thus, the grid of tube 2 is shown connected by aconductor 12 to a point in the anode circuit between the negativeterminal of source B" and the resistor 11.

The action of the volume control tube 10 is very wellknown. As thesignal field strength decreases below a predetermined value, there is alesser flow of current through the anode circuit of the device. In otherwords, the potential drop across the resistor 11 becomes less with theresult that the bias on the grid of tube 2 is diminished. The capacity11 is shunted across the resistor 11 to by-pass radio frequency cur-Therefore, it will be seen that the amplification is increased. Thereverse action takes place as the signal field strength increases abovea predetermined level.

In other words, the device 10 functions to regulate, or control, thegain of the amplifier tube 2 (and other amplifier tubes if desired) inan automatic manner in accordance with the variation of the signal fieldstrength. The magnitude of the source 0 is such that with no radiofrequency input (that is when no station is being received) there is azero drop of potential across the resistor 11.

Obviously at such times the radio frequency tubes are not amplifyingsignal energy. Background noises become amplified to a disconcertingextent. In general, it has been found that it is highly desirable toconsiderably diminish the background noises when the received fieldstrength is not high relative to the noise level. Since, as alreadypointed out, the particularly annoying noises reside in the upper audiofrequency range, this can be accomplished by attenuating the high audiofrequencies whenever the signal field strength decreases below a desiredvalue. By arranging the attenuation mechanism to be automatic in itsaction, and designing the mechanism so that little, or no, attenuationis introduced at signal field strengths greater than a specific value,for example 200 microvolts, effective suppression of background noisesis secured.

Specifically, the attenuation mechanism comprises a space discharge tube13 having its control electrode connected, by a conductor 14, to a pointon the conductor 12. That is to say, the bias for the tube 13 isproduced by the drop across the resistor 11, as in the case of the tube2. The anode circuit of tube 13 includes a bypass capacity 15 connectedbetween a high potential point in the anode circuit of tube 13 and asimilar point in the anode circuit of detector tube 5. The capacity hasa low impedance to the high audio frequencies, and is shunted by a chokecoil 16 having a high impedance to high audio frequencies.

It will be appreciated that, from an electrical viewpoint, there is avariable impedance connected in series with capacity 15, the impedance.

being the anode impedance of tube 13. With change in bias on the grid ofthe latter, the value of the anode impedance will vary. Obviously, thisvariation is not only dependent on the operation of the automatic volumecontrol tube, but functions in an automatic manner to regulate theby-passing of high audio frequencies through the capacity 15.

The operation of control tube 13 will, then, be such that there is aminimum by-passing through capacity 15 when the drop across the resistor11 is a maximum thereby increasing the bias on the. grid of tube 13 withthe result that the series anode impedance is a maximum. The by-passingof high audio frequencies through capacity 15 is at a maximum when thedrop across the resistor 11 is a minimum (that is zero, when no stationsare received) thereby decreasing the bias on the grid of tube 13 withthe result that the series anode impedance becomes sufficiently great toblock the capacity 15.

It will, therefore, be seen that there has been provided a radioreceiver arrangement for automatically discriminating against relativelyhigh audio frequencies in accordance with received signal fieldstrength. Furthermore, it should be clear that the arrangement employedherein comprises a variable impedance path cooperating with an automaticgain control device to attenuate high audio frequencies atpre-determined periods in the operation of a radio receiver.

While I have indicated and described one arrangement for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationshown and described, but that many modifications may be made withoutdeparting from the scope of my invention as set forth in the appendedclaims.

What I claim is:

1. In combination, a radio frequency amplifier stage including anelectron discharge tube, a detector stage including a space dischargetube having its input electrodes coupled to the output electrodes of theamplifier tube, a gain control circuit including a space discharge tubehaving its input electrodes coupled to the input circuit loo of thedetector tube, means connecting the grid circuit of said amplifier tube.to the output circuit of said gain control tube, an attenuation circuitincluding a space discharge tube having its grid circuit connected tothe output circuit of said gain control tube, a path of low impedance torelatively high audio frequencies between the anode circuit of saidattenuation tube and the anode circuit of said detector tube, and a pathof high impedance to said audio frequencies in shunt with said firstpath.

2. In combination, a radio frequency amplifier stage including anelectron discharge tube, a detector stage including an electrondischarge tube having its input electrodes coupled to the output circuitof the amplifier tube, an automatic gain control circuit for saidamplifier stage including an electron discharge tube having its inputelectrodes connected to said detector stage, means connecting anelectrode of said amplifier tube to the output circuit'of said gaincontrol tube, an attenuation circuit including an electron dischargetube having its grid circuit connected to the output circuit of saidgain control tube, a condenser of low impedance to relatively high audiofrequencies connected between the anode circuit of said attenuation tubeand the anode circuit of said detector stage, and an .inductance of highimpedance to said audio frequencies in shunt with said condenser.

3. In combination, a radio frequency amplifier adapted to have audiomodulated high frequency energy impressed upon its input; a rectifierhaving its input connected to the output of said amplifier, meansresponsive to variations in the intensity of the energy impressed uponsaid amplifier input to vary the transmission efficiency of saidamplifier in a direction to maintain constant the intensity level ofsaid modulated high frequency energy supplied to said rectifier, and acapacity path including a control tube means connected directly betweenthe anode of the rectifier and ground, and responsive to said amplifiertransmission efiiciency control means, for by-passing to ground thehigher audio frequency components of the rectifier output energywhenever the transmission efiiciency of said amplifier increases above apredetermined level.

4. In combination, a detector tube circuit, a source of signal energy tobe detected coupled to the detector circuit input, an attenuatingcircuit in shunt across the detector tube output circuit, saidattenuating circuit including in series a space discharge device and aninductance coil of high impedance to high audio frequencies, and meansfor controlling the conductivity of said device in response tovariations in signal energy amplitude derived from the detector tubeinput circuit.

5. In combination, a detector tube circuit, a

source of signal energy to be detected coupled to the detector circuitinput, an attenuating circuit in shunt across the detector tube outputcircuit, said attenuating circuit including a space discharge device andan inductance coil of high impedance to high audio frequencies, acondenser, across-said coil, which has a low impedance to said highaudio frequencies, and means for controlling the conductivity of saiddevice in response tovariations in signal energy amplitude derived fromthe detector tube input circuit.

6. In combination, a detector tube circuit, a source of signal energy tobe detected coupled to the detector circuit input, an attenuatingcircuit in shunt across the detector tube output circuit, saidattenuating circuit including in series a space discharge device and aninductance coil of high impedance to high audio frequencies, and anelectronic relay means for controlling the conductivity of said devicein response to variations in signal energy amplitude derived from thedetector tube input circuit.

7. In a signal energy detector tube network, a transformer couplingmeans connected between the detector anode and cathode, an audiofrequency by-pass path connected in shunt with the anode and cathode ofthe said tube, said path including a condenser of low impedance tohigher audio frequencies and a tube, one side of said condenser beingconnected to one of the transformer windings, and a by-pass auxiliarytube, connected between the detector input circuit and the controlelectrode of said by-pass tube, for controlling the conductivity of thelatter.

8. In a signal energy detector tube network, an audio frequency Icy-passpath connected in shunt with the anode and cathode of the said tube,said path including a condenser of low impedance to higher audiofrequencies and a by-pass tube, and a choke coil in shunt with thecondenser, and an auxiliary tube, connected between the detector inputcircuit and the control electrode of said by-pass tube, for controllingthe conductivity of the latter.

WILLIAM S. BARDEN.

